The features and their description are readily available at other sites so I will concentrate on things other than that.
I was looking at the 88 key PX-310 Privia, found a good price on this unit, and bought it instead. It is roughly equivalent to the PX-410R without the hammer action weighted keyboard. The 8000 also includes a wood stand.
Casio has made tremendous strides in improving their sound chip. They are not quite as good as Yamaha and Roland (professional series) but the gap is rapidly closing and they definitely give you the most bang for your buck. The Hammond organ sounds are particularly well done and you can literally set up almost any organ sound on this keyboard that you could on a real drawbar organ. You get four drawbar volume positions instead of eight but otherwise, about equivalent. The percussion even mutes if you have a key being held when you strike additional keys (like a real Hammond).
There are 620 sounds on the WK-8000, 3800, 3300, PX-410R. That's not 620 samples or waveforms, of course. They have taken the basic sounds (a bit more than the general MIDI set but not near 600) and done many of the things with them that you might want yourself but not know how to do. Add tremolo, chorus, or phase shift to the electric piano, add wah-wah or phase shift to the clavinet, add octave to the brass, strings, etc. When you want an electric piano, it's nice to be able to choose from 18 or 30 instead of three.
Editing is simple. What most of us do is splits and layers and they are quick and can be set up "on the fly". Using the mixer, you can pitch shift each end of the keyboard SEPARATELY, something most of the Privia series will not do. That means you can play a middle C in the low range for a piano and still have a normal brass range up in the higher end. Also, you can do both splits and layers simultaneously and even mute an element while playing. That means you can have a piano and string in the right hand and just a bass in the left (you mute the layer channel for the left hand). Then if you "unsplit" by pushing split, your string AND piano both go all the way to the bottom of the keyboard. The mixer will readily change the volume of the bass, the strings . . . any channel you wish. These are features that are very musically useful for me. I love my Yamaha S-80 (an older unit with a slightly superior piano sound) but cannot edit it while playing. When you go to edit on that one, you only hear the channel being edited.
The piano sound (the one item that essentially defines the quality of a piano-type keyboard) is the best Casio has had to date. It uses their 32 note ZPI sound source (that I BELIEVE is triple sampled on any unit it is used on by definition) which is their latest multi-hundred sound chip (at this writing). They have a new 128 note chip (API) but it appears to be available only on digital pianos - units that limit themselves to relatively few non-piano sounds.
One of the features becoming more and more popular in these keyboards is built-in speakers. Even Yamaha is building professional level keyboards with on-board speakers. That's incredibly useful for practice and any home or small room gigs. And the quality is excellent. The 8000 is very, very large but also very light. The size is probably exclusively to provide room for the bass reflex enclosure and the sound shows it. It's 12 watts which is slightly above par for a self-contained keyboard. Casio is the first, by the way, to have broken the 30 pound mark on weighted keyboards (which the WK-8000 is NOT). Their PX series mostly weigh 26 pounds with a graded hammer weighted action. That's also what the 8000 weighs without the weighted keys. Note that the only difference between this one and the WK-3800 is number of keys (88 vs. 76). The WK-3300 adds a 3.5" floppy drive (which I'm certain is still there only because they must have had a lot of them left on hand) and takes away the line outs and mod wheel (gives you a on-off mod pushbutton instead). Modulation control is ESSENTIAL on a Casio if you want to be able to start and stop the "Leslie" on the organ sounds. That's the only way of doing it without changing patches. The pushbutton on the 3300 is more than sufficient for this purpose, by the way. The 8000 has the mod wheel so no worries. The speed change includes the very realistic ramp up or down on the rotor speed, just like the real thing. You don't hear the dual rotors but everything else is right on.
MIDI jacks are going away on these lower end units but they give you USB instead. I see all of the Casio models I've mentioned as being positioned somewhere between home keyboards and professional units. I have frequently gigged with a Casio but generally use Yamaha, Alesis and Korg.
Anyway, it is incredible how much synthesizer power your dollar can buy today through products like Casio's WK workstations and their PX digital pianos. And you can't buy a cheaper Hammond simulator.